Summary: Astrobiologists have a strong interest in understanding the conditions on the early Earth, but the record for the first 700 million years of Earth history is gone. The stages that made our planet fit for life are not recorded in the rocks we have today.

Building a habitable Earth

Astrobiologists have a strong interest in understanding the conditions on the early Earth, but the record for the first 700 million years of Earth history is gone. The stages that made our planet fit for life are not recorded in the rocks we have today.

The Earth was not a habitable planet when it first formed: it was a seething cauldron of molten material, with impact material raining down as the young Earth swept up those remnants of the solar disk that fell within its gravitational influence. In order to recount the history of life on Earth, we will first need to understand the structure of the planet at the end of this accretion era.

Francis Albarède, of the Ecole Normale Supérieure in Lyon, France, has pointed out that we know nothing about the initial mineral composition and structure of the Earth. That’s because mountain building and erosion have destroyed the most ancient geological records. The oldest terrestrial material is a mineral – a zircon – dated at 4.4 billion years. The most ancient rock you can hold in your hand is 4.1 billion years old. And if you want to take a stroll across early Earth, you must go to Isua, Greenland where there is a swath of continental crust dated at 3.8 billion years.

Recovering the record of the early Earth owes much to a decade of rapid progress in which geologists have learned to use radioactive rocks as natural clocks. For example, the decay of uranium into lead allows scientists to determine the age of a rock with a precision of less than one million years (if the rock still contains some uranium). As a result of such improvements in chronology, our picture of Earth’s formation is becoming more clear.

The first planetary objects in our solar system probably formed 4.568 billion years ago. It took just a few million years of collisions to change the dust in the solar nebula into objects up to 60 miles in size. The oldest meteorites, chondrites, survive from that era. Collisions continued, building ever larger objects by consolidation and fusion. By looking at the radioactive decay of hafnium to tungsten, geologists have concluded that the formation of Earth’s molten core was probably completed in 30 million years.

What was Earth like just 30 million years after the onset of planetary formation? The outer layer, known as the mantle, was molten as a result of vigorous bombardment from objects raining down. Our planet was entirely covered by an ocean of magma, or semi-liquid rock. (Volcanic lava is a modern example of magma.)

“Without a stable continental crust, you cannot have life as we know it,” says Hervé Martin of the Observatoire de Physique du Globe in Clermont-Ferrand, France, who studies the oldest rocks and the magma that generated them. Martin and his colleagues have derived the conditions of pressure and temperature under which the ancient rocks formed, and determined that the magma led to the formation of basalt. “Later the melting of basalt gave rise to the granite that made the continents,” says Martin. “The question is, where did these melting and formation processes take place?”

To answer that question, Martin and his colleagues have turned to analogues of the early Earth that exist today on the surface of our planet. They study subduction zones, regions where the ocean crust is being sucked back into the mantle. It’s only in subduction zones that you find temperature and pressure conditions similar to those on early Earth. What Martin’s research has uncovered is that the formation of granite, and continents, occurred when basalt melted in subduction zones on the primitive Earth.

This research on early Earth has important connections with astrobiology. Plate tectonics, which endlessly remold the surface of our planet, were more active in the past when the plates were smaller. Crucially, a planet with active tectonics is able to distribute the mineral and chemical nutrients required for the development of life.

It is well-known that terrestrial life requires water and a source of energy, and in this connection the zircons tell an interesting story. Micro-analysis of their crystals reveals that the early continental crust was stable enough and extensive enough to escape complete obliteration as cosmic debris continued to bombard the Earth. Examination of the oxygen isotopes in zircon suggests that material in the Earth’s crust was interacting with water. The implication is that Earth probably had liquid water and perhaps even oceans on its surface as early as 4.4 billion years ago.

Although astrobiologists now have a reasonable understanding of the origin of the continental plates and the importance of tectonics for the origin of life, the same claim cannot be made for the oceans. Water does not leave much of a geological signature. According to Daniele Pinti of the Université du Québec in Montréal, Canada, astrobiology has two central questions. First, how and when did water arrive on Earth? And second, when did internal heat flow decrease sufficiently to allow liquid water to be stable on the surface?

The scientific community generally agrees that water arrived at Earth during the final stage of accretion from the solar nebula. There are differences of opinion on the mode of transport. Some have proposed impacts of comets as the source of water, an idea that excites astrobiologists because comets are also a great resource of the organic molecules needed for the bricks of life. On the other hand, others in the community propose asteroidal impacts. What unites the two camps is agreement that the water came from the outer solar system by way of comets or asteroids.

The final aspect that needs to be considered when building a habitable Earth is the origin of its atmosphere. Here the situation is even more uncertain. The first atmosphere would have had a composition similar to that of the Sun, in which hydrogen dominates. But Earth’s gravity was too weak to retain the hydrogen, so it leaked away into space. As is the case with water, to find the source of our atmosphere an extraterrestrial origin beckons. Cosmic dust may be responsible for the nitrogen, which comprises some 80% of the atmosphere today. There are strong suggestions too that the carbon required for carbon dioxide must have come from meteoritic impacts.

The consensus opinion by Martin and his colleagues is that the early Earth was completely unsuited to fostering life as we know it. Life could only get a foothold after the Earth had cooled down, the continents had formed, and the volatile elements that had evaporated during the violent processes of Earth’s formation had been replenished by those couriers of the solar system: comets, asteroids, and meteorites.